#24 Need to validate data type of PropertyDescriptors

[Dave Depinet]

Currently in ModelEditor, if a PropertyDescriptor has a
getter/setter that don't exist, a validation error is
displayed. However, if the datatype is wrong, no
validation message is given.

The validation on PropertyDescriptor should be improved:

1) Validate that the field exists (may be in a base class)
2) Validate the field's data type (if specified). Warn if
it's not the same.
3) Validate the attributes like public/private, etc.
4) Validate that the getter/setter exist.
5) Validate the data type of the getter/setter.

In addition, we need the ability to switch from a
ReferenceDescriptor back to a PropertyDescriptor.
(Currently can only go one way.)